1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring formation parameters by transmitting and receiving electromagnetic signals by means disposed in recesses in a tubular housing member and including means for reducing the capacitive coupling of noise from conducting elements located adjacent the recess and housing means. More particularly, a method and apparatus is disclosed employing electrostatic shielding of the antennas and ground loop isolation to reduce noise and minimum spacing of antennas from conductive elements adjacent the recesses and inductive current coupling means to enhance the signal level at the receiving means whereby the system is capable of use in a drill string to make measurements while drilling.
2. Description of the Background
It is desirable for many reasons to transmit electrical signals through the earth as a propagating medium, and receive the signals at a location spaced from the transmitter. Such a signal propagation system is, for example, used both for the determination of various parameters associated with the propagating medium and for communication purposes. These systems are often used in the investigation of the environment surrounding a borehole, and in particular, the surrounding earth formations. Various types of borehole logging systems are available to perform these investigations. A class of these systems utilize electromagnetic field phenomena to obtain data from the environment surrounding the borehole.
One type of electromagnetic logging is electrode logging which utilizes an electric field in the surrounding formation to produce a measure of the conductivity of the formation. A conductive mud is necessary for proper use of this system, thus rendering the system inoperative with oil base muds. Inductive logging is another type of electromagnetic logging which uses a magnetic field in the formation to produce a secondary current flow in the formation. The secondary current flow sets up a second magnetic field which induces current in receiving coils positioned in the borehole. The induced current in the receiving coil or coils is proportional to the secondary current flow in the formation and thus is directly proportional to the conductivity or inversely proportional to the resistivity of the surrounding formation. Electromagnetic wave propagation affords still another logging system for investigating the environment around a borehole and is the subject of the present invention.
An electromagnetic logging system of the wave propagation type is disclosed in Gouilloud et al, U.S. Pat. No. 3,551,797, which is incorporated herein by reference. This patent discloses a wireline system having a transmitter and receivers for measuring formation parameters, and utilizing phase comparison and amplitude. However, the Gouilloud wireline system is not usable in a measuring while drilling (hereinafter, "MWD") configuration. The Gouilloud patent discloses a non-conductive sonde of insufficient strength to operate in a drill string characterized by a mass of steel and more particularly drill collars in the vicinity of the drill bit and measurement apparatus. U.S. Pat. Nos. 4,107,597 and 4,185,238 also show electromagnetic wave propatation systems for use in wireline apparatus. U.S. Pat. No. 4,107,597 describes the wireline sonde as being constructed of a non-conductive material which is customary in such devices in order to accommodate the use of electromagnetic transmitting and receiving apparatus. The U.S. Pat. No. 3,079,550 shows an induction logging system for measuring similar formation parameters, utilizing lower frequencies and requiring a conductive mud in the borehole.
Both the electrode and induction systems, heretofore the primary methods for measuring formation resistivity, as well as the wireline systems using wave propagation, have certain drawbacks, particularly for application in a MWD configuration. An electrode system requires insulation of the drill string from the several transmitting and receiving electrodes in the system. This normally requires a special insulation coating to be applied over the steel drill string in the vicinity of the electrodes. This coating is expensive to maintain and is of questionable reliability. An induction logging system normally operates at 20 KHz and requires large diameter coils to obtain the necessary coupling. In a MWD configuration, inductive logging coils must be mounted in or about a drill collar in a drill string and that portion of the collar must be non-conductive. Non-conductive collars are difficult to build while maintaining the structural integrity and strength necessary to their use in a drill string. In this regard thicker collar walls and improved mechanical strength characteristics are obtainable in a collar by reducing the size of the coils. In order, however, to achieve the necessary coupling between spaced coils which are small, the operating frequency of the system must be increased. As the frequency is raised from 20 KHz, wave propagation begins and standard induction is no longer effective. The wireline electromagnetic wave propagation devices described above do not use a sonde assembly having sufficient structural integrity and strength to be incorporated in a drill string or noise reduction and signal enhancement techniques which enable reliable performance in an MWD environment.
The unsuitability of the above systems for incorporation into a drill string to measure formation parameters using electromagnetic signals in a measurement while drilling configuration is clear. The electrode system discussed above is limited to use with electrically conducting, water base muds. Induction logging systems utilize large coil configurations which require too much space on a drill collar to maintain the strength and fluid communication characteristics described above. Smaller coils require the use of higher frequencies to insure proper coupling between coils, and the higher frequencies propagate in the formation. Thus, standard induction measurements cannot be utilized in MWD systems. The lack of structural integrity and ineffective signal reception are problems associated with these systems. The art has long sought a means of overcoming these disadvantages useful in providing an effective apparatus using electromagnetic signals to measure formation parameters while drilling.